In the technology of automation and process control, field devices are used in many cases in the flow path of an industrial process for measuring (sensors) process variables or controlling (actuators) controlled variables.
Field devices for determining flow, fill status, pressure difference, temperature, etc. are generally known. For detecting the corresponding process variables mass, or volume, flow rate, fill level, pressure, temperature, etc., the field devices are most often arranged in the immediate vicinity of the relevant process component.
The field devices deliver a measured signal, corresponding to the value of the detected process variable. This measured signal is forwarded to a control unit (e.g. a programmable logic controller PLC, a control room or process control system PCS). Normally, the process control is accomplished by a control unit, where the measured signals of various field devices are evaluated and, on the basis of the evaluation, control signals are produced for the actuators, which control the course of the process.
In terms of an example of actuators, controllable valves can be mentioned, which regulate the flow rate of a liquid or a gas in a section of a pipeline.
Signal transmission between field device and control unit can occur in analog or digital form (e.g. current loop or digital data bus). Known international standards for signal transmission include 4-20 milliampere current loops, HART®, Profibus®, Foundation Fieldbus' or CAN-bus®.
The signal processing in the field device and the communication of the field device with the control unit or other field devices is becoming always more complex. To handle this, various hardware components with corresponding software are being implemented in the field device. The software, which runs as a sequential program in a microprocessor, is normally very flexible and can easily be replaced. The disadvantage in the use of software is that the data processing proceeds sequentially and, for this reason, it is relatively slow.
Hardware components, in contrast, have a determined functionality, which is hardwired in special chips, or integrated circuits (IC's). Examples of such are ASIC's (Application Specific Integrated Circuits) or SMD's (Surface Mounted Devices). These devices are very application-specific and can, for example, execute an FFT (Fast Fourier Transformation), which is very calculations intensive, very quickly. The disadvantage of these hardware components is that they are only flexible to a slight degree and normally must be replaced, in order to achieve a changing of the functionality.
The communication of the field device with a superordinated, evaluation unit occurs, likewise, over suitable hardware components partially still in analog fashion or over a digital data bus.
Each field device is normally composed of various hardware components, which determine the functionality of the field device. Different field devices, for instance Coriolis mass flowmeters or electromagnetic flowmeters (MID's), have entirely different hardware components. Even for one and the same field device, for example a Coriolis mass flowmeter, the hardware components for communications can differ, for example. For connection to a Profibus, a Profibus module is needed or connection to a Foundation Fieldbus, a Foundation Fieldbus-module, etc. Depending on whether the field device is to deliver a frequency, pulse or current signal, corresponding hardware components have to be provided.
This multiplicity of components means a considerable expense in manufacture, since a multitude of hardware components has to be available.
A trend in the case of field devices is that they should be always compacter. The components, especially the hardware components, are, therefore, always moving closer together on the circuit boards. A limit has almost been reached for this.
In order to assure the safety and the ability of a field device to function, the hardware components must be tested following the populating of the circuit boards. For test strategies to this point in time, a multitude of test pads are provided on the underside of the circuit board. These pads can be contacted using a so-called bed of nails. In this case, only certain circuit parts can be tested in isolation.
If, in the field, a Coriolis mass flowmeter is to be replaced by an electromagnetic flow meter, it is presently necessary to replace the entire field device.